Short-term exposure to ambient fine particulate matter (PM2.5) is an important risk factor for increased morbidity and mortality in older adults. The number of Americans over age 65 is expected to reach 70 million by 2030, making it imperative to better understand how to reduce the impact of PM2.5 in communities where a growing number of older adults reside. Seniors spend a significant portion of their time indoors, and yet few studies have examined how personal, indoor, and outdoor PM2.5 levels - measured concomitantly - are associated with cardiovascular (CV) health effects. In addition, little information is available on the effectiveness of air filtration as a way to mitigate personal exposures to ambient PM2.5 and reduce air pollution-related health effects, an important question for highly susceptible elderly individuals. The proposal seeks to conduct a randomized crossover intervention study with repeat health and exposure measurements on 40 seniors who are exposed to unfiltered, low-efficiency (LE) filtered, and high-efficiency (HE) filtered air. Dat will be collected in a senior citizen facility in Detroit, Michigan, in an area where we have previously linked outdoor PM2.5 sources to CV health effects including blood pressure changes and impaired endothelial function. The central hypotheses are that indoor air filtration will significantly reduce personal PM2.5 exposure and improve CV health; and compared to outdoor PM2.5 levels, indoor and personal PM2.5 levels will more accurately predict adverse CV responses among residents living in a senior-citizen facility. Specific Aims are: (1) to determine whether filter-based interventions are capable of reducing PM2.5 exposures and thereby producing improvements in clinically-relevant intermediate CV health endpoints in a senior-citizen residential facility; (2) to identify which measurement of PM2.5 - outdoor, indoor or personal - is most closely associated with intermediate CV health outcomes under three different filtration scenarios in a senior-citizen residential facility; and (3) to determine whethr filter-based interventions are capable of reducing exposures to specific outdoor sources of PM2.5 and thereby improving clinically-relevant intermediate CV health endpoints in a senior-citizen residential facility. The proposed study is expected to generate new insights on (1) the effectiveness of economical LE filters and HE filters for reducing personal PM2.5 exposures and improving CV health, (2) the relative accuracy of three different exposure metrics as predictors of CV health endpoints, and (3) which outdoor sources and their associated CV health effects are most responsive to filtration. This intervention study has the potential to reduce the incidence of CV events in this and other highly susceptible populations.